This invention is related to the subject matter of U.S. Pat. Nos. 3,427,871, and 3,673,861 issued Feb. 18, 1969 and July 4, 1972, respectively, which describe methods and apparatus for testing soil. The present invention pertains to improvements over these prior devices and methods. More specifically, this invention relates to a rock borehole shear tester.
Tester of the prior art have used laterally expandable shear plates. However, the shear plates were incapable of lateral movement as shear forces were applied to accommodate irregularities in the bore. Further, prior shear heads could not accommodate changes in soil volume during shear (i.e., "dilatancy"). The shear head of this invention can permit limited movement of the expandable shear plates during shear. The pivot points in the head allow it to expand or contact slightly as the rock changes in volume during shear.
The shear plates on the device of this invention are mounted upon expandable equalizer plates which are held parallel during initial expansion to allow use in variable size holes without introducing force. The lock is released upon application of the shearing stress, and resets automatically at the conclusion of a test.
The shear plates are rotatable inserts, for easier replacement and to better conform to roughness in the hole. The shear plates are preferably pushed, rather than pulled, and this is accomplished by mounting the shear head in the bottom of the pulley cradle which is pulled upwardly by conventional apparatus. Only one test may be conducted at each contact, but as many as four tests may be performed at the same depth by rotating the shear head and pulling cradle. Uniform pressure is exerted on the shear head by a pressure block that engages the expanded shear head after the pressure plates have engaged the rock, and after the shear force has been applied.